Allow pass notes to send_email

[ricardopolo]

When you create a new email using send_email() it creates a brand new transaction.
If you need some information in that transaction like notes filled in another hooks you cannot access to them.

This PR allows to pass a new option the send_email() to preserve the notes. It just adds another option to the options object that send_email() already receives.

Includes updated docs.
Fixes #1293

[codecov-io]

Current coverage is 36.98%

Merging #1295 into master will decrease coverage by -0.06% as of fa18956

@@            master   #1295   diff @@
======================================
  Files           31      31       
  Stmts         5750    5753     +3
  Branches      1313    1315     +2
  Methods          0       0       
======================================
- Hit           2130    2128     -2
  Partial        225     225       
- Missed        3395    3400     +5

Review entire Coverage Diff as of fa18956

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[msimerson]

+1

Absent any further discussion (here or in #1293), I'll merge this after 2015-01-21.

[smfreegard]

+1 - seems sensible. I meant to merge it the other day, but forgot.