Various bug fixes and enhancements (version 0.59a)

Fixed bug whereby incoming gcodes were URI-decoded twice, which could
cause problems if a comment contain a % character
Fixed bug whereby M109 and M190 waited for all heaters to reach target
temperature instead of just the one involved
Fixed bug whereby files whose names contain an uppercase letter G could
not be uploaded or deleted
Increase buffer size for incoming gcodes
Removed parameter to set max PWM value in M106 command. Max PWM value is
now 255 as per recent RepRapPro change.
Changed the code that deals with dropping characters in unimportant
messages sent to USB such that we only drop characters at the end of a
line (adding ".\n" to whatever we send) or drop whole lines
Added binary 059a-dc42