Maintain redirect options in session. Fix #221

To reflect redirect_to options, keep redirect url in session to be set
'X-XHR-Current-Location' in response header.
Use X-XHR-Redirected-To instead of X-XHR-Current-Location

Set redirect location to X-XHR-Redirected-To header
only when server redirects user.
And use the location info in reflectRedirectedUrl.
Merge pull request #222 from YasuOza/keep_anchor_on_redirect

Maintain redirect options in session. Fix #221
Reflect changes from PR #222
Escape URLs when checking for cross-origin redirects
handle server errors (5xx)
append changelog with note about this fix
Merge pull request #228 from keram-refinery/handle_5xx_error_v2

handle server errors (5xx)
version 1.2.0
improving readability and performance

small refactoring by @dhh suggestion
comments by @reed
Merge pull request #231 from keram-refinery/refactoring

improving readability and performance
Update README.md
Merge pull request #238 from kathryneh/patch-1

Fix typo
Update README.md to fix overflowing code snippet

The "Asset change detection" code snippet overflowed from the 
content area, so I added a newline in the middle of it and made
it be a code block.
Merge pull request #240 from VesQ/patch-1

Update README.md to fix overflowing code snippet
Add link to Flask Turbolinks
Record changes from PR 231
Add the page:before-change event
Make cache size configurable
Rename cacheSize to pagesCached