2.4 fix json parser e+ bug

According to http://www.json.org/ the double in JSON can be represented in scientific notation, and e symbol can be one of the following:
e
e+
e-
E
E+
E-
net.liftweb.json.parse was working ok with e, e-, E, E- but was throwing exception "can't parse" for e+ and E+.
Added unit test (was failing). Added c == '+' analogous to c == '-' to JsonParser.scala. This fixed unit test.
P.S. The same issue exists in master, but we use 2.4 so I fixed it in 2.4 branch. The change is atomic and can be merged to any lift version, including master.