Make possible to build library without curl and rsa signature -related files

add --with-curl flag
add preprocessor macroses, that
	allow to build without Service constructor, that uses default sendRequest
	allow to build completely without RSA_SHA1 signature
Update README.md, to describe --with-curl flag usage
Fix exception message in Signature class

fix exception message for unsupported signing method in Signature class
Fix README.md

README.md restructured
Use #ifdef in Service interface instead of throwing exception
Merge pull request #42 from swat32exe/make-curl-optional

Make curl and rsa signature optional