Enhancement to allow changing encoding of attachment, and various bug fixes

Enhancement:

• Adds ability to pass encoding (modified addAttachmentPart, addAttachmentPartFromFile with additional optional parameter), #98

Bug fixes:

• "UT" timezone is not correctly handled by PHP #97 (thanks @pupaxxo )
• Don't treat an incorrect header the same as a correct one (contenttype is not the same as content-type if both are set as headers) #102
• Use normalized header name when generating header object based on header name #102