avoid download large response

introduce DOWNLOAD_MAXSIZE and DOWNLOAD_WARNSIZE in settings and
download_maxsize/download_warnsize in spider/request meta, so
downloader stop downloading as soon as the received data exceed the
limit. also check the twsisted response's length in advance to stop
downloading as early as possible.
add 2 more test cases and minor doc fixes
Force to read DOWNLOAD_TIMEOUT as int (for example to pass using
environment variable)
pin mitmproxy 0.10.1 as >0.11 does not work with tests
mitmproxy 0.10.1 needs netlib 0.10.1 too
attemp to fix travis fails
Merge pull request #946 from tpeng/limit-response-size

avoid download large response
Merge pull request #954 from kalessin/int-download-timeout

Force to read DOWNLOAD_TIMEOUT as int (for example to pass using environment variable)
An attempt to resolve #977, add signal to be sent when request is dropped by the scheduler
#977 - test dropping requests
Merge pull request #961 from ldmberman/task_977_request_dropped

An attempt to resolve #957, add signal to be sent when request is dropped by the scheduler
fixed the variable types in mailsender documentation
Merge pull request #976 from aufziehvogel/fixed_mailsender_vartypes

fixed the variable types in mailsender documentation
S3DownloadHandler: fix auth for requests with quoted paths/query params
Merge pull request #978 from immerrr/fix-s3-authorization-for-quoted-paths

S3DownloadHandler: fix auth for requests with quoted paths/query params
t.i.b.ThreadedResolver is now a new-style class

since 2009 / twisted-9.0.0
https://github.com/twisted/twisted/commit/663d669dce6ee5009eee8c6d2a81f5199855178b
Merge pull request #987 from nyov/nyov/resolver

t.i.b.ThreadedResolver is now a new-style class
Merge pull request #1 from scrapy/master

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