Added extra documentation to add_mhtml_content regarding encoding

* Remind clients using add_mhtml_content to encode or replace entities
  when passing in text directly.
* Added tests for POD, Converter and add_mhtml_content to verify
  expected behaviour.
Validate encodings and set codepage of generated eBook

* Only allow ISO-8859-1 or UTF-8 in set_encoding
* Translate encoding to codepage and pass to Mhtml2Mobi to set in MOBI
  header.
* Tests to verify that generated file has correct codepage
Remove file write/read in save method

This should no longer be needed if encodings and codepage are set
correctly.
Merge pull request #26 from rupertl/utf8

Encodings and codepages proposed changes for issue #25