coding std
updated README.md
improved exceptions
fix joining a table via different columns

Since target table can be joined via different columns from source table
or it can be joined from different tables via column with the same name,
there is a key consisting of source table, target table and referencing
column, which uniquely describes a join.
2 args where accepts explicitly specified comparison operator
stupid replacing of placeholders (only for '2 args where' now)
is NULL in 2 args where
changed query api
added authorship
fixed pattern in variable parsing
slightly revisited exceptions
reset context after applying query
LeanMapperQuery\Query::applyQuery: no need to explicitly specify table
name

Now $fluent is parsed to get source table name.
improved registering commands
possibility to use query object in entity
typos
degraded nette/tester
first tests
fixed recognizing of DateTime
updated tests