PHP DocBlocks, Constants Changes, and Minor Refactors

What I Changed
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DocBlocks
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- PHP docblocks have been added to all classes, functions, and member variables
- This is in reference to this pull request https://github.com/mailgun/mailgun-php/pull/55 
- Added link references to class docblocks of Mailgun, MessageBuilder, and BatchMessage

Constants
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- Deleted the Constants file
- Moved the constants from the deleted file to two new classes
    - `Mailgun\Constants\Api` for Api related constants
    - `Mailgun\Constants\ExceptionMessages` for exception messages
- Refactored other classes that used the old constants to now use the new constant classes
- Messages/Values were not changed
- Did this so that I could create new instances of the MessageBuilder class without going through the Mailgun client. I also feel like it is a little cleaner to "use" the constants on the files that will need them rather than depend on being instantiated through a different class.

Minor Refactors
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- Removed unnecessary constructor
- Removed a function return from `finalize()` method on BatchMessage class. The `sendMessage()` function used does not return anything.
- Removed unused class imports and one unused member variable

Testing
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I have tested this locally with simple Mailgun message sending (which was successful). I have also tested with the BatchMessage and MessageBuilder. Everything seems to be working fine, however *I have not tested all functionality within the package*. The changes I made were minor for the most part, however any feedback or comments from any other eyes would be helpful.

If there are any questions or comments please let me know.