Improved diagnostics and documentation

• Introducing check command which performs basic sanity checks of the environment, such as presence and permissions of the directories. check is also executed as part of adding or removing trust.
• Certificate files are being check to contain single certificates when adding. This is to avoid silently allowing addition of certificate bundles: they will not (and should not) be handled properly for trust purposes.
• An implementation of c_rehash command is included with clrtrust. It removes dependency on external c_rehash. External c_rehash will be used if present, but is no longer required. Option -c|--internal-rehash has been added to force using the internal implementation.
• README.md is added, providing friendly description of the tool on github frontpage.
• Miscellaneous bug fixes.