Make #extract_schema_and_table an instance method in Utils

Also, move the utils test into its own test case.
fix exists? to return false if passed nil (which may come from a missing URL param)
fix assert_select_email to work on non-multipart emails as well as converting the Mail::Body to a string to prevent errors.
Do not try to use as a parameter elements that are not acceptable
incorporate feedback from vijaydev and dasch to rephrase this to sound more natural, and some grammar fixes.
fixed 500 error message which is misleading by suggesting all rails apps have exception notifications by default
remove misleading line on 500 error page.
Fixed typos and made minor changes in the Plugins guide.
Fixed bad options order in ActiveRecord::Migration::CommandRecorder#invert_rename_index
Documentation fixes
Split long string into multiple shorter ones

This makes the code more readable.
Extract simplified_type into the abstract class
Move the bulk alter table code into the abstract mysql adapter, hence it is supported for mysql2 as well now.
The join_nodes must be passed to the JoinDependency initializer and therefore counted by the alias tracker. This is because the association_joins are aliased on initialization and then the tables are cached, so it is no use to alias the join_nodes later. Fixes #2556.
Move clear_timestamp_attributes into Timestamp module
Merge pull request #2021 from dasch/more_postgres_utils

Make #extract_schema_and_table an instance method in Utils
Merge pull request #2720 from biow0lf/master

Doc fixes
Fix test failures on 1.8.7, since Object#initialize_dup is not defined there (and this call to super is no essential, so easiest to simply remove)
let SDoc add a link to the source code in GitHub for each method
Document the Sprockets compressors

Add documentation to Sprockets::NullCompressor and
Sprockets::LazyCompressor. Also, make LazyCompressor#compressor
private, as it isn't part of the public API.