s/Postmark/SendGrid

Fix copy/paste error in exception message for failed api calls
Fix bug in double referencing response array value
Remove array dereferencing to support php 5.3
Use HTTPS for API access
Create explicit exception messages

Name the service inside the exception message. The exception class
is used between different services, so name the service to
understand the service where it came from (otherwise, the stack
trace is the only option).

Also, remove the exception messages where only the response of the
api call is used. Give the message more context to use a fixed part
and a variable response part in the message.
Parse XML response and implement error code checks
Small fixes to get email sending working
Remove Mac OS X files
Fix typo s/stange/strange
Use SendGrid 4xx/5xx http codes to handle errors
Use interface as type hint instead of abstract class
Fix typo s/functionnalities/functionalities
Use relative links to the README
Create factory for HTTP client

Various developers might need to tune the options of the HTTP
client. SlmMail gives access to the HTTP adapter and the options
via a configuration key.

The HTTP client is now created with a factory and injected in the
abstract service. If the client is not injected, the service is
still capable of instantiating a default HTTP client itself.
Add AlphaMail service
Add pricing table
Place slash in api call (analoque to other services)
Bump version to 1.0.0, add @bakura10 as author
Add link to AWS module
Revert docs to use php 5.3 again