Gsm alphabet, 7bit message concatenation 

Hey I made 4 modifications (in 4 different commits)
-Use of GSM alphabet instead of ASCII for 7bit decryption. This allows in particular to use characters such as 'é', 'à', etc... with 7 bit
-bug fix: Remove the last character (undefined) when receiving 16bit message
-Use of GSM alphabet instead of ASCII for 7bit encryption
-The 7-bit messages concatenation was not working for me (only 16bit was working). And this is because you need to make padding after the header, as explained here http://en.wikipedia.org/wiki/Concatenated_SMS I recoded the encode7Bits to make it work

I tested all the 4 of them