feat/Provide an option to download libraries for MockVault or real network

[bochaco]

This provides three options for downloading the safe_app lib (in all cases the version is fixed and hard-coded in the package.json just like it's been so far):
1- npm install will download the safe_app lib which connects with the real network
2- NODE_ENV=dev npm install will download the safe_app lib which connects with the mocked vault (the published lib's filename shall have -dev postfix)
3- NODE_ENV=nightly npm install will download the nightly build of the safe_app lib (the published lib's filename shall have -nightly postfix)

[coveralls]

Coverage remained the same at 84.749% when pulling 9abafab on bochaco:downloading-dev-or-network-libs into 572ccfb on maidsafe:master.

[coveralls]

Coverage remained the same at 84.749% when pulling 9abafab on bochaco:downloading-dev-or-network-libs into 572ccfb on maidsafe:master.

[coveralls]

Coverage remained the same at 84.749% when pulling ae06bc1 on bochaco:downloading-dev-or-network-libs into 367dcc3 on maidsafe:master.

[coveralls]

Coverage remained the same at 84.749% when pulling ae06bc1 on bochaco:downloading-dev-or-network-libs into 367dcc3 on maidsafe:master.