use more appropriate std::runtime_error instead of std::logic_error

fixed warnings found by cppclean

.gitignore updated

worker: make safer fork & exec logic

refactoring: use more c++11x features

admin tool: use readline library, if it's possible

prun: use multi-line print for help command

version up: 0.15

config param rename: ipv6 -> ipv6_only

PEP8 code style fix

admin tool: removed extra '> ' print

master, common: print error code description; master: destroy partially initialized IHistory object

docs: renamed section 'External dependencies' -> 'Optional external dependencies'

docs: mentioned '/etc/pworker/worker.cfg'

docs: added 'External dependencies' section in README