Mathops extensions

Add a new `math_rpn` function, and capacity to add new operators when using the RPN parser.
New operators (RPN only): `exp`, `floor`
Built README
Corrected typos.
Rebuilt README
mediaproxy: always parse To header
mediaproxy: fixed detecting messages without a body
Fixed crash when dr_state_flusher() (via timer) may be called before data is loaded (by first SIP worker)
Credits go to shimaore on GITHUB
Fixed copy'n'paste error in the state flushing routine.
Reported by Stéphane Alnet (shimaore)
Closing issue #154
Fixed bugs with processing m4 in fedora init scripts
Merge pull request #158 from nikbyte/master

Fixed bugs with processing m4 in fedora init scripts
Correct an error message
Merge pull request #144 from shimaore/mathops_extension

Mathops extension: new RPN expression parsing function