DB backend, improve & doc: start documenting + think about standardization
DB backend, improve & doc: improve new documentation
DB backend, improve & doc: reduce parameters on compile_selector()
DB backend, improve & doc: move handling of set statements to compile_db_selects()
DB backend, improve & doc: redesign

- redesign representation of parent selectors
- db.compile_selector() does not accept 'statement', but 'selector'
  instead
- remove unnecessary function arguments
DB backend, improve & doc: re-design db.merge_conditions()

- db.merge_conditions() is called separately for each object type -
  object type is no longer passed, only list of conditions
Compile DB Selects: export function compile_selectors_db
DB/osm2pgsql: bugfix objects_by_id(): don't include add. columns in tags
DB/osm2pgsql: fix objects_member_of, correct config key
DB/*: make objects_* functions ready for bulk processing; follow new docu
Compile DB Selects: bugfix, create selector clones via deepcopy
Compile DB Selects: when resolving statements, prohibit duplicate statements
Compile DB Selects: first build list of selectors, then compile
Compile DB Selects: check if next selector is sub selector of any other
Compile DB Selects: check if next selector is master selector of others
Compile DB Selects: check sub selectors: handle has_tag conditions
Merge pull request #93 from plepe/db-improve-document

DB backend API: add documentation & improvements