Swift Package Manager and Linux support

[novi]

Added Package.swift and Linux support. It passes all tests on macOS and Linux.

And I have made changes to Xcode based project(for Cocoapods, Carthage):

• add BitcoinKitXcode compiler flag to change import declaration

BitcoinKit.xcodeproj will conflict with Swift Package Manager generated xcodeproj. But it not affects for usage but for development.

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   52.17%   52.17%           
=======================================
  Files          47       47           
  Lines        3241     3241           
  Branches       58       58           
=======================================
  Hits         1691     1691           
  Misses       1546     1546           
  Partials        4        4

Impacted Files	Coverage Δ
Sources/BitcoinKit/GetBlocksMessage.swift	0% <ø> (ø)
Sources/BitcoinKit/ByteStream.swift	0% <ø> (ø)
Sources/BitcoinKit/InventoryMessage.swift	0% <ø> (ø)
Sources/BitcoinKit/ServiceFlags.swift	0% <ø> (ø)
Sources/BitcoinKit/GetDataMessage.swift	0% <ø> (ø)
Sources/BitcoinKit/AddressMessage.swift	0% <ø> (ø)
Sources/BitcoinKit/Script.swift	54.54% <ø> (ø)
Sources/BitcoinKit/HDWallet.swift	0% <ø> (ø)
Sources/BitcoinKit/BloomFilter.swift	43.24% <ø> (ø)
Sources/BitcoinKit/PaymentURI.swift	82.22% <ø> (ø)
... and 36 more

---

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[usatie]

Hi, @novi !
Thanks for dropping us this PR.
I am one of the new maintainer of this project.

And I am planning to merge our BIG PR #69 first, but it will surely conflict a large part of you PR.
I'm sorry, can you create the PR again after we merged #69 ?

P.S.
I decided to merge this PR first.
So please just resolve the current conflict!

[usatie]

@GitCash send 5000 bits to @novi .
Thanks for dropping us a PR!
This is just a small tip!

[GitCash]

Hey novi, user usatie tipped you 5000 bits in Bitcoin Cash ( ~ $3.005 ).

Click here to claim it!

You can also add the "thumbs down" reaction to usatie's comment above to prevent future tips.

[usatie]

@novi now I could resolve the conflict between novi:work/package-swift and yenom:master.
So can you resolve the current conflict and push the change?
I will merge this PR first, and then, I'll resolve the conflict between BitcoinCashKit:master and yenom:master!

[novi]

Hello. I will check them later.

[usatie]

@novi Hi, thank you for your help!

[usatie]

@novi Thanks for dropping us PR, we already merged your branch to this branch, so will close this PR.
c01df11

I thought your commits going to be included, but it turn out to be not...
So sorry for that.