store: Added sdb_store_get_host().

… querying a host by it's name.
frontend: Added simple implementation of sdb_fe_exec().

This function executes the return value of sdb_fe_parse().
frontend parser: Added 'len' parameter to sdb_fe_parse().

When greater than (or equal to) zero, this parameter let's the parser parse a
substring of the specified query. This makes it easier to pass in a command
stored in a connection's input string buffer.
frontend: Implemented the CONNECTION_QUERY command.

This command expects a string representation of the to be executed command as
the command body. The frontend will then parse the string and execute the
first command. Other commands are (currently?) ignored to make injection
attacks harder.
parser_test: Test partial query parsing.
sysdb: Append newlines to lines read from readline().

Else, whitespace will be missing in multi-line commands.
Added t/cibuild.sh and use that in the Travis configuration.

This allows for some more flexiblity.
.travis.yml, t/cibuild.sh: Test builds with libedit and libreadline.

Using an env variable for this purpose, thus, making both options show up in
the Travis build matrix.