Warning: no migration from old xml cache provided yet

 - Add content type information in LIME encrypted message
 - Single entry point for all types of chat messages that are first decrypted if necessary and then handled according to their content type
 - Add possibility to send chat messages with a content type that is not 'text/plain'
 - Encrypt IMDN

Added callbacks in im encryption engine for decrypting incoming files in chat messages + improved lime test to check decrypted file is equal to sent one

Added belle_sip_request_t parameter to im encryption engine callbacks to authorize read/write of SIP headers

- enabled by default switch using --enable-lime
- check for polarssl gcm.h presence (version >=1.3)

+ test for the encrypted text message
+ fix bug introduced in last merge

- add a boolean into linphoneCore to check if encryption is requested
- move encryption/decryption from chat.c to sal_op_message.c
- check encrypted message validity
- return error to application when enable to encrypt/decrypt

- not fully functional yet.